They’d prefer to show their ad to flower enthusiasts who
frequent their website or subscribe to their newsletter. To get
the special offer to those people who are also on Twitter, the
shop may share with us a scrambled, unreadable email address (a
hash) or browser-related information (a browser cookie ID). We
can then match that information to accounts in order to show them
a Promoted Tweet with the Valentine’s Day deal. This is how most
other companies handle this practice, and we don’t give
advertisers any additional user information.

This is similar to Facebook's retargeting features in FBX,
All Things D notes, although Twitter doesn't have an ad
exchange.